The present invention relates to a magnetic random-access memory (MRAM) having a magnetic substance structure.
An MRAM is a memory device which utilizes a phenomenon that the resistance to an electric current flowing through a magnetic substance structure varies depending on the direction of electron spin (magnetization direction) in the magnetic substance. A tunneling magnetoresistive (TMR) element is an example of an element conventionally used for performing information storage operation. Each cell of a magnetic memory, capable of storing one bit of information, is made of one each TMR element and metal-oxide-semiconductor (MOS) transistor.
FIG. 16 is a cross-sectional diagram showing the construction of a conventional MRAM device. In this Figure, designated by the numeral 100 is a TMR element having a sandwich structure with a thin insulator layer 102 sandwiched between a first magnetic substance layer 101 and a second magnetic substance layer 103. Designated by the numeral 150 is a semiconductor substrate (hereinafter referred to simply as the substrate) on which an access transistor, which is a MOS transistor, is formed, and designated by the numeral 155 are source/drain regions of the access transistor. Designated by the numeral 160 is a readout word line which serves as a gate electrode of the access transistor and designated by the numeral 165 is its write word line. Designated by the numeral 170 is an electrode section for connecting one of the source/drain regions 155 to the first magnetic substance layer 101, designated by the numeral 171 is another electrode section of the other source/drain region 155, designated by the numeral 175 is an interlayer insulator stacked between the individual layers, and designated by the numeral 180 is a bit line. In this structure, the first magnetic substance layer 101 forms a free-spin layer in which the direction of electron spin is unfixed and variable while the second magnetic substance layer 103 forms a fixed-spin layer in which the direction of electron spin is fixed to a specific direction. Since the sandwich structure of the TMR element 100 has a rectangular shape in top view that is elongate in the direction of the bit line 180, the spin direction in the first magnetic substance layer 101 could easily become parallel to the direction of the bit line 180 (bit line direction). The spin direction in the second magnetic substance layer 103 is fixed to this bit line direction.
Storage (writing) of data into the TMR element 100 of the aforementioned conventional MRAM device is performed by producing flows of electric currents through the bit line 180 and the write word line 165 and determining the spin direction in the first magnetic substance layer 101 which forms a free-spin layer with the aid of magnetic fields generated by the electric currents, as shown in FIG. 17. Specifically, a binary xe2x80x9c1xe2x80x9d or a binary xe2x80x9c0xe2x80x9d is written in the TMR element 100 depending on whether the spin direction in the first magnetic substance layer 101 is the same (parallel) as or opposite (antiparallel) to that in the second magnetic substance layer 103. This data write operation requires a specific magnetic field strength to perform. In addition, the data write operation is characterized in that it is carried out in one memory cell where the corresponding bit line 180 and the corresponding write word line 165 intersect each other.
On the other hand, the data stored in the TMR element 100 is read by applying a voltage across the first magnetic substance layer 101 and the second magnetic substance layer 103 and a voltage to the readout word line 160 to turn on the access transistor and then measuring an electric current flowing into the access transistor. The amount of this electric current is large when the spin direction in the first magnetic substance layer 101 is the same as that in the second magnetic substance layer 103, whereas the amount of the electric current is small when the spin direction in the first magnetic substance layer 101 is opposite to that in the second magnetic substance layer 103. This property is used in the execution of data read operation. Specifically, the data in the TMR element 100 is read by varying the electrical resistance between the first magnetic substance layer 101 and the second magnetic substance layer 103, turning on the access transistor, and judging the amount of the electric current flowing from the bit line 180 into the access transistor.
In the aforementioned conventional MRAM device, multiple electrically conductive layers including TMR elements are arranged, forming a multilayer structure with the interlayer insulator 175 or other elements placed between them. In addition, the conventional MRAM device requires separately arranged write word lines and readout word lines. The structure of memory cells which is perpendicular to the plane of the substrate 150 is so complex that its manufacturing processes are complicated and it has been difficult to reduce the area of each cell.
This invention is intended to provide a solution to the aforementioned problems of the prior art. Accordingly, it is an object of the invention to simplify the structure of memory cells and thereby provide a magnetic random-access memory which enables further miniaturization, a higher level of integration and easier production compared to the prior art.
In a first principal form of the invention, a magnetic random-access memory comprises a semiconductor substrate on which a first word line and a bit line intersecting each other are arranged, a tunneling magnetoresistive element formed in an area of intersection of the first word line and the bit line by stacking a first magnetic substance layer whose magnetization direction is variable, a second magnetic substance layer whose magnetization direction is fixed and an insulator layer placed between the first and second magnetic substance layers, and an access transistor which uses as a gate a second word line extending in a direction intersecting the bit line, wherein the bit line is made of ferromagnetic metal whose magnetization direction is fixed to the longitudinal direction of the bit line, and the bit line acts also as the second magnetic substance layer.
This arrangement of the invention is advantageous in that it serves to simplify the structure of the magnetic random-access memory and facilitate its manufacture.
In one aspect of the invention, the magnetization direction of the bit line is fixed to its longitudinal direction only in its portion where the bit line intersects the first word line.
This makes it possible to easily create unidirectional magnetic domains in which the spin direction is aligned in a single direction as well as to improve the reliability of the magnetic random-access memory.
In a second principal form of the invention, a magnetic random-access memory comprises a semiconductor substrate on which a first word line and a bit line intersecting each other are arranged, a tunneling magnetoresistive element formed in an area of intersection of the first word line and the bit line by stacking a first magnetic substance layer whose magnetization direction is variable, a second magnetic substance layer whose magnetization direction is fixed and an insulator layer placed between the first and second magnetic substance layers, and an access transistor which uses as a gate a second word line extending in a direction intersecting the bit line, wherein the tunneling magnetoresistive element is located in an area of intersection of the second word line and the bit line, and the second word line acts also as the first word line.
This arrangement of the invention is advantageous in that it significantly reduces the complexity involved in creating a multilayer interconnection structure, simplifies the structure and manufacturing processes of the magnetic random-access memory, and enables its miniaturization and a higher level of integration.
In one aspect of the invention, the magnetic random-access memory of the aforementioned second principal form is operated in such a manner that a voltage of a polarity which does not turn on the access transistor is applied to the word line when writing data in the tunneling magnetoresistive element by flowing electric currents through the bit line and the word line, and a voltage is applied to the word line to turn on the access transistor without flowing an electric current through the word line when reading the data from the tunneling magnetoresistive element by flowing an electric current from the bit line to the access transistor.
This makes it possible to perform data read and write operations in a reliable fashion.
In another aspect of the invention, the magnetic random-access memory of the aforementioned second principal form is operated in such a manner that a voltage of a polarity which turns on the access transistor is applied to the word line such that an electric current flows through the word line with a specific voltage gradient when writing data in the tunneling magnetoresistive element by flowing electric currents through the bit line and the word line, whereby the data can be read from the tunneling magnetoresistive element as the data is written therein.
This makes it possible to write the data while verifying that the data is successfully written.
The construction of the magnetic random-access memory of the aforementioned second principal form may be such that one of source/drain regions of the access transistor is connected to the tunneling magnetoresistive element while the other of the source/drain regions is connected to a diffusion layer, which is formed on the semiconductor substrate adjacent to the aforesaid other of the source/drain regions and serves as a well contact for taking out the potential of the semiconductor substrate.
This also makes it possible to reduce the complexity involved in creating a multilayer interconnection structure, simplifies the structure and manufacturing processes of the magnetic random-access memory, and enables its miniaturization and a higher level of integration.
The construction of the magnetic random-access memory of the aforementioned second principal form may be such that a silicide layer bridging surfaces of the diffusion layer serving as the well contact and its adjacent source/drain region is formed by a salicide process.
This further simplifies the structure of the magnetic random-access memory and helps achieve its further miniaturization.
In a third principal form of the invention, a magnetic random-access memory comprises a semiconductor substrate on which a first word line and a bit line intersecting each other are arranged, a tunneling magnetoresistive element formed in an area of intersection of the first word line and the bit line by stacking a first magnetic substance layer whose magnetization direction is variable, a second magnetic substance layer whose magnetization direction is fixed and an insulator layer placed between the first and second magnetic substance layers, and an access transistor which uses as a gate a second word line extending in a direction intersecting the bit line, wherein the first and second word lines are electrically connected to each other.
This arrangement makes it possible to apply control operation used in a structure employing a common word line which serves both as the first and as the second word lines.
In a fourth principal form of the invention, a magnetic random-access memory comprises a semiconductor substrate on which a word line and a bit line intersecting each other are arranged, a tunneling magnetoresistive element formed in an area of intersection of the word line and the bit line by stacking a first magnetic substance layer whose magnetization direction is variable, a second magnetic substance layer whose magnetization direction is fixed and an insulator layer placed between the first and second magnetic substance layers, and a diode having a pn junction and connected to the tunneling magnetoresistive element, wherein data stored in the tunneling magnetoresistive element is read by flowing an electric current from the bit line through the diode.
Since the access transistor is not needed in this arrangement, it is possible to even more simplify the structure of the magnetic random-access memory and achieve its further miniaturization and a higher level of integration.
The construction of the magnetic random-access memory of the aforementioned fourth principal form may be such that the diode is formed of a well region of a first conductivity type created in the semiconductor substrate and a diffusion layer of a second conductivity type created within the well region and connected to the tunneling magnetoresistive element, wherein the data stored in the tunneling magnetoresistive element is read by flowing the electric current from the bit line through the diode in its forward direction.
This makes it possible to realize a simplified structure requiring no access transistor in a reliable fashion.
The construction of the magnetic random-access memory of the aforementioned fourth principal form may be such that the diode is formed of the first magnetic substance layer and a conductor layer which forms a Schottky junction or a metal-to-metal pn junction together with the first magnetic substance layer, wherein the data stored in the tunneling magnetoresistive element is read by flowing the electric current between the bit line and the diode in its reverse direction overwhelming a junction breakdown voltage.
This also makes it possible to realize a simplified structure requiring no access transistor in a reliable fashion.
The construction of the magnetic random-access memory of the aforementioned fourth principal form may be such that the word line is located above the bit line.
This serves to increase the degree of freedom in device pattern design and achieve its further miniaturization and an even higher level of integration.
These and other objects, features and advantages of the invention will become more apparent upon reading the following detailed description in conjunction with the accompanying drawings.